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GCN Circular 36355

Subject
X-ray transient EP240414a: Detection of associated supernova
Date
2024-04-30T21:29:28Z (18 days ago)
From
Andrew Levan at Radboud University <a.levan@astro.ru.nl>
Via
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A.J. Levan (Radboud), J. van Dalen (Radboud), P. Jonker (Radboud), D.B. Malesani (DAWN/NBI & Radboud), J. Quirola-Vasquez (Radboud), F.E. Bauer (PUC) report for a larger collaboration: 

We have obtained further monitoring of the counterpart of EP240414a (Lian et al. GCN 36091, Aryan et al. GCN 36094) with the VLT, GTC, SOAR, and NOT. The optical source has decayed to an approximately constant i-band magnitude of i~22 but has declined more rapidly in the g-band, with the g-i colour changing from ~0.4 at four days to g-i~2 at 15 days post-burst. The current magnitudes and colours are consistent with a supernova component now visible in the light curve. 

Furthermore, an X-shooter spectrum obtained on 25 April 2024 is not the power-law spectrum expected of a GRB afterglow and shows, at low signal-to-noise, features consistent with a broad-lined type Ic supernovae at z~0.4, the redshift of the putative host galaxy (Jonker et al. GCN 36110). Based on the combined light curve and spectral evidence, we suggest that EP240414a is associated with a massive star collapse at a relatively unusual location within its host galaxy. 

We also note that the peak luminosity and lightcurve shape over the time frame 2-8 days after the transient onset consists of a rise and fall whose shape and luminosity are reminiscent of the fast blue optical transient AT2018cow. However, limited X-ray coverage makes tracking the afterglow behaviour challenging. 

We thank the staff of the NOT, GTC, SOAR and the VLT for their excellent support.

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